1. Field of the Invention
The present invention relates to an axial fan unit including two axial fans arranged in series.
2. Description of the Related Art
Electronic devices such as personal computers or servers commonly include a cooling fan to cool electronic components contained in a case thereof. As high-density mounting of the electronic components inside the case advances, improved performance of such cooling fans has been demanded. In particular, for use in comparatively large electronic devices such as servers, cooling fans that produce an air flow with high static pressure and high air volume have been desired.
An exemplary technique for achieving increased static pressure in cooling fans is to arrange two axial fans in series to form a fan unit. For example, Japanese Patent No. 3,717,803 discloses a configuration of two impellers arranged in series in an axial direction along a rotation axis.
However, such conventional serial axial fan units suffer a problem of decreased air volume and static pressure, as energy loss occurs when a flow of air produced by the upstream fan enters into the downstream fan.
In the case of a serial axial fan unit including two axial fans with the same air volume and static pressure characteristics arranged in series along the rotation axis (i.e., the two axial fans are substantially coaxial with each other), for example, a maximum static pressure (i.e., a static pressure when the air volume is zero) is expected to be twice as high as it is when there is only one axial fan. In practice, however, the maximum static pressure is only about 1.5 times as high, and experiments have shown that, even with stationary vanes provided between the upstream fan and the downstream fan, the maximum static pressure is only about 1.8 times as high.
In conventional serial axial fan units, the upstream fan and the downstream fan are arranged to rotate in the same direction. In this case, velocity components of the air flowing from the upstream fan toward the downstream fan include a whirl component, i.e., a velocity component in the same direction as that of rotation of the upstream fan. This means that the air flowing into the downstream fan has velocity components including a whirl component in the same direction as that of rotation of the downstream fan. This means that a rotation speed of the downstream fan relative to the flow of the air decreases, resulting in a failure of the downstream fan to act on the air to a sufficient degree. This can be considered to be a factor in the failure to sufficiently improve the static pressure characteristics.
In the serial axial fan unit disclosed in Japanese Patent No. 3,717,803 the downstream fan and the upstream fan are arranged to rotate in different directions. As such, this serial axial fan unit is not designed to allow the downstream fan to perform a sufficient job on the flow of the air caused by the rotation of the upstream fan when the downstream fan and the upstream fan rotate in the same direction.